Variable Valve Activation (VVA) mechanisms for internal combustion engines are well known. It is known to lower the lift, or even to provide no lift at all, of one or more valves of a multiple-cylinder engine, during periods of light engine load. Such deactivation or valve lift switching can substantially improve fuel efficiency.
Various approaches are known for changing the lift of valves in a running engine. One known approach is to provide an intermediary cam follower arrangement, which is rotatable about the engine camshaft and is capable of changing the valve lift and timing, the camshaft typically having both high-lift and low-lift lobes for each such valve.
For example, a Roller Finger Follower (RFF) typically acts between a rotating camshaft lobe and a pivot point such as a Hydraulic Lash Adjuster (HLA) to open and close an engine valve. By way of example, switchable deactivation RFF includes an outer arm, also known as body or low-lift follower, and an inner arm, also known as high-lift follower. The inner arm supports a roller carried by a shaft. The roller is engaged by a lobe of an engine camshaft that causes the outer arm to pivot about the HLA, thereby actuating an associated engine valve. The deactivation RFF is selectively switched between a coupled (high-lift) and decoupled (zero-lift) mode. In the coupled mode the inner arm is coupled to the outer arm by a movable latching mechanism and rotation of the lifting cam is transferred from the roller through the shaft to pivotal movement of the outer arm, which in turn, reciprocates the associated valve. In the decoupled mode, the inner arm is decoupled from the outer arm. Thus, the inner arm does not transfer rotation of the lifting cam lobe to pivotal movement of the outer arm, and the associated valve is not reciprocated. In this mode, the roller shaft is reciprocated within the outer arm.
A switchable, two-step RFF operates in a manner similar to the deactivation RFF, as described above. However, one particular difference between the operation of a deactivation RFF and a two-step RFF occurs in the decoupled mode of operation. When in the decoupled (zero-lift) mode, the outer arm of a deactivation RFF may be engaged by zero-lift cam lobes and remains in a static position allowing the associated valve to remain closed. On the other hand, when in decoupled (low-lift) mode, the outer arm of a two-step RFF is engaged by low-lift camshaft lobes to thereby reciprocate the associated engine valve according to the lift profile of the low-lift camshaft lobe.
A lost motion spring maintains contact between the roller and the lifting portion of the camshaft lobe when either type of RFF (i.e., deactivation or two-step) is in the decoupled (zero-lift or low-lift, respectively) mode and absorbs the reciprocal motion of the shaft and roller. The lost motion spring biases the inner arm away from the outer arm of the RFF. The expansion force of the lost motion spring acting on the inner arm must on the one hand be sufficient to maintain contact of the roller with the lifting portion of the cam lobe, while on the other hand must not cause the HLA, which supports the outer arm to be pumped down by the force of the lost motion spring.
Another known approach is to provide a deactivation mechanism in the Hydraulic Lash Adjuster (HLA) upon which a cam follower rocker arm pivots. Such arrangement is advantageous in that it can provide variable lift from a single cam lobe by making the HLA either competent or incompetent to transfer the motion of the cam eccentric to the valve stem. Yet another known approach is to provide a deactivation mechanism in the Hydraulic Valve Lifter (HVL).
During the operation of the above mentioned two-mode variable valve activation devices a variety of failure modes may occur. One failure mode of particular concern is the condition when one or more of the two-mode variable valve activation devices are stuck in the low-lift or zero-lift mode. This failure mode may have severe base-engine-level consequences at high engine speeds since the lost motion spring is only able to absorb the force provided by the lobe of the camshaft to the roller up to certain engine rotational speeds. Thus, extensive mechanical failure of the engine may occur if the engine is operated at high engine speeds in low-lift or zero-lift mode. Currently used passive diagnostic strategies that rely upon existing data available in engine management systems are in many cases neither responsive nor sensitive enough to satisfy customer requirements. The only alternative presently available is to compromise the camshaft profile to reduce valve closing velocity, thereby reducing the destructive energy associated with running the engine at high speeds in low-lift or zero-lift modes. This alternative is unacceptable because the resultant camshaft profile negates most of the potential fuel economy benefits achieved by applying two-mode VVA to the engine. This situation hampers the ability of the original equipment manufacturers to provide a two-mode VVA, a proven fuel economy and emissions improvement technology, in a federally certified production vehicle.
What is needed in the art is the ability to reliably detect a low-lift or zero-lift failure mode that occurs when one or more two-mode variable valve activation devices are stuck in low-lift mode at high engine speeds where these devices typically operate in high-lift mode.
It is a principal object of the present invention to provide a method and apparatus for direct measurement of the mode of each two-mode variable valve activation device used in a multiple-cylinder engine.